1. Technical Field of the Invention
The present invention relates to a charger, and particularly to a charger having a plurality of wires used as charging electrodes which are covered by a channel-shaped shield member.
2. Brief Description of the Related Prior Art
For example, in an image forming apparatus of an electrophotographic system, a corona charger is widely used as a means for charging the surface of a photoconductive member. Further, a charger is conventionally known which is provided with a plurality of corona wires so that an increased charging power, necessary for a high speed operation of an image forming apparatus, can be obtained.
In these chargers, a scorotron in which a grid electrode is provided in front of a corona wire and potential is applied to the grid electrode for controlling the charging power or a corotron having no grid electrode is used. A shield member surrounding the corona wire is used as a holder for removably mounting the corona wire in the image forming apparatus.
On the other hand, a charger is conventionally known in which exhaust openings for sucking and exhausting ozone generated by corona discharge therethrough are provided in a shield member surrounding three sides of a corona wire. As shown in FIG. 1, in a charger 20 in which a plurality of corona wires 21a, 21b are provided in a shield member 22, a plurality of elongated exhaust openings 23 are arranged with an adequate space therebetween in parallel with the corona wires 21a, 21b.
However, when ozone is sucked through the exhaust openings 23 so as to be exhausted outside, dust such as dispersed toner and paper powder in the machine is also sucked and discharged through the shield member 22. As a result, such dust attaches to the corona wires 21a, 21b, and portions of the corona wires 21a, 21b in opposition to the exhaust openings 23 are especially soiled, so that these portions are caused to have a low discharging power. The soiled and therefore low discharging portions of each of the corona wires 21a, 21b are in the positions adjacent to the exhaust openings 23, and in these positions the photoconductive member is insufficiently charged. As a result, the photoconductive member is charged differently in two kinds of portions. Those are the portions in opposition to the exhaust openings and the portions not in opposition to the exhaust openings, and after development, lines are produced on a recording sheet.
When compulsory exhaustion of gases around a charger is not executed, such a problem as above-mentioned is caused by air discharged from the apparatus or an air stream produced by a cooling fan.